The use of optical fibers in signal transmission systems has prompted the use of optical fiber attenuators that allow high-powered transmission signals to be processed directly into fiber optic receivers. Fiber optic receivers are incapable of processing incoming optical signals if those signals have a power level that is too large. The maximum acceptable power level is a property of each receiver and is known as the upper limit of that receiver's dynamic range. Because the optical loss of fiber optic systems may vary, provisions must be made to allow the optical power level delivered to the receiver to be modified.
Fiber optic attenuators are devices which reduce the amount of power present in an optical signal. Fiber optic variable attenuators are fiber optic attenuators in which the amount of this reduction may be adjusted.
In the past, fiber optic variable attenuators have involved, for example, the use of lenses and variable neutral density optical filters (e.g. as described in U.S. Pat. No. 4,989,938), the variation of the relative orientation the ends of two fibers (e.g. as described in U.S. Pat. Nos. 4,645,294 and 4,519,671), and movement of a reflective surface, thereby effecting change of the coupling between two fibers (e.g. as described in U.S. Pat. No. 4,516,827).
These techniques are labor intensive, especially when applied to singlemode fibers, in that very precise alignment of the two fibers is necessary to ensure a low loss when the device has been adjusted to provide a minimum of attenuation. In addition, costly optical components, such as lenses and filters, are used in the construction of the majority of currently available variable fiber optic attenuators. In general, many prior art devices are expensive and complex, imprecise in control, and susceptible to external shocks and vibrations.